System and process for flushing residual fluid from transfer lines in simulated moving bed adsorption

ABSTRACT

A process according to various approaches includes flushing an intermediate transfer line between a raffinate stream transfer line and a desorbent stream transfer line away from the adsorptive separation chamber to remove residual fluid including desorbent from intermediate transfer line. The process may include directing the residual fluid flushed from the intermediate transfer line to a recycle stream to introduce the residual fluid into the adsorptive separation chamber.

FIELD OF THE INVENTION

The subject invention relates to a process for flushing residual fluidfrom a transfer line in an adsorptive separation process for separatinga preferentially adsorbed component from a feed stream. Morespecifically, the invention relates to a system and process for flushingresidual desorbent fluid from a transfer line prior to withdrawing araffinate stream through the transfer line during the continuoussimulated countercurrent adsorptive separation of aromatic hydrocarbons.

BACKGROUND OF THE INVENTION

Para-xylene and meta-xylene are important raw materials in the chemicaland fiber industries. Terephthalic acid derived from para-xylene is usedto produce polyester fabrics and other articles which are in wide usetoday. Meta-xylene is a raw material for the manufacture of a number ofuseful products including insecticides and isophthalic acid. One or acombination of adsorptive separation, crystallization and fractionaldistillation have been used to obtain these xylene isomers, withadsorptive separation capturing a great majority of the market share ofnewly constructed plants for the dominant para-xylene isomer.

Processes for adsorptive separation are widely described in theliterature. For example, a general description directed to the recoveryof para-xylene was presented at page 70 of the September 1970 edition ofCHEMICAL ENGINEERING PROGRESS (Vol. 66, No 9). There is a long historyof available references describing useful adsorbents and desorbents,mechanical parts of a simulated moving-bed system including rotaryvalves for distributing liquid flows, the internals of the adsorbentchambers and control systems. The principle of using a simulated movingbed to continuously separate the components of a fluid mixture bycontact with a solid adsorbent is as set forth in U.S. Pat. No.2,985,589. U.S. Pat. No. 3,997,620 applies the principle of thesimulated moving bed to the recovery of para-xylene from a feed streamcontaining C₈ aromatics, and U.S. Pat. No. 4,326,092 teaches meta-xylenerecovery from a C₈-aromatics stream.

Adsorptive separation units processing C₈ aromatics generally use asimulated countercurrent movement of the adsorbent and the feed stream.This simulation is performed using established commercial technologywherein the adsorbent is held in place in one or more cylindricaladsorbent chambers and the positions at which the streams involved inthe process enter and leave the chambers are slowly shifted along thelength of the beds. A typical adsorptive separation unit is illustratedin FIG. 7 and includes at least four streams (feed, desorbent, extractand raffinate) employed in this procedure and the location at which thefeed and desorbent streams enter the chamber and the extract andraffinate streams leave the chamber are simultaneously shifted in thesame direction at set intervals. Each shift in location of the transferpoints delivers or removes liquid to or from a different bed within thechamber. In general, to simulate countercurrent movement of theadsorbent relative to the fluid stream within the chamber, the streamsare shifted in the general direction of fluid flow, i.e. the downstreamdirection, within the chamber to simulate the solid adsorbent moving inthe opposite, i.e. upstream, direction. The lines at these transferpoints are reused as each stream enters or leaves the associated bed,and each line therefore carries one of the four process streams duringsome point of the cycle.

The art recognizes that the presence of residual compounds in thetransfer lines can have detrimental effects on a simulated-moving-bedprocess. U.S. Pat. Nos. 3,201,491; 5,750,820; 5,884,777; 6,004,518; and6,149,874 teach the flushing of the line used to deliver the feed streamto the adsorbent chamber as a means to increase the purity of therecovered extract or sorbate component. Such flushing avoidscontamination of the extract stream with raffinate components of thefeed remaining in this line when it is subsequently used to withdraw theextract stream from the chamber. U.S. Pat. No. 5,912,395 teachesflushing of the line just used to remove the raffinate stream in orderto avoid contaminating feed with raffinate when this line is used todeliver the feed stream to the adsorbent chamber. All of thesereferences teach flushing such lines back into the adsorbent chamber,thus increasing the separation load within the chamber. U.S. Pat. No.7,208,651 discloses flushing away from the adsorbent chamber thecontents of a transfer line which previously has been used to remove theraffinate stream with one or both of a feed mixture and a materialwithdrawn from the adsorption zone. The residual raffinate within thetransfer line is flushed to join the raffinate stream as feed to araffinate column. U.S. Pat. No. 6,149,874 discloses flushing residualfeed from a common section of fluid distribution piping to a boostercircuit.

One previous exemplary system s utilized up to three flushes to handleresidual fluid remaining in the transfer lines. A primary flushdisplaced residual extract from the transfer line just used to removethe extract stream with fluid from the desorption zone of the chamberjust below the desorbent stream and directed it through a rotary valveto a transfer line just used to inject the feed stream. Because thevolumes in the transfer lines were about equal, theextract-plus-desorbent fluid displaced the residual feed that hadpreviously been in the transfer line into the adsorbent chamber justabove the current feed stream position so that the residual feed couldbe separated with the feed stream within the adsorptive separationchamber and to avoid contamination of the extract stream with theresidual feed remaining in the transfer line when the extract streamsubsequently shifted to the transfer line previously occupied by thefeed stream. Further, the residual extract from the primary flush usedto displace the feed remained in the transfer line to be subsequentlywithdrawn by the extract stream to increase yield of the extractproduct.

The exemplary system sometimes included a secondary flush. The secondaryflush utilized a flush of fluid, typically desorbent, through thetransfer line and into the chamber immediately below the extract line.The secondary flush provided a “wash” of this transfer line with thedesorbent to minimize the amount of contaminates, including raffinate,feed, and other components that may remain in the transfer line afterthe primary flush so that these materials were not withdrawn from thetransfer line with the extract. Because this transfer line waspreviously flushed with desorbent and extract via the primary flush, thesecondary flush was typically used in applications requiring high purityextract. The secondary flush would push the extract and desorbentmaterial previously in the transfer line back into the adsorptiveseparation chamber. The secondary flush is an optional flush utilized tomeet high purity demands of the extract product.

In some systems, a tertiary flush was also utilized. The tertiary flushincluded a flush of the transfer line previously occupied by theraffinate withdrawal stream. The tertiary flush was utilized to removethe residual raffinate from this transfer line to restrict thisraffinate from being injected back into the adsorbent chamber with thefeed upon subsequent arrival of the feed stream to the transfer line.Because the raffinate stream is depleted of the desired extractcomponent, the tertiary flush was carried out so that the residualraffinate was not injected back into the adsorptive separation chamber,which would otherwise increase the separation demands in order to removethis additional raffinate material. The tertiary flush was accomplishedby flushing the transfer line away from the adsorptive separationchamber with fluid from a port of the chamber adjacent to the transferline.

SUMMARY OF THE INVENTION

According to various approaches, a process is provided for separatingcomponents in a feed stream by simulated countercurrent adsorptiveseparation. The process includes introducing a feed stream and adesorbent stream into two different ports via two differentcorresponding transfer lines along a multi-bed adsorptive separationchamber. The feed stream has at least one preferentially adsorbedcomponent and at least one non-preferentially adsorbed component. Themulti-bed adsorptive separation chamber has plurality of beds that areserially connected in fluid communication and comprising a predeterminednumber of spaced ports with corresponding transfer lines in fluidcommunication therewith for introducing and removing fluid into and fromthe adsorptive separation chamber. The process also includes withdrawingan extract stream and raffinate stream through two different ports ofthe multi-bed adsorptive separation chamber via two differentcorresponding transfer lines. The process according to this approachincludes flushing an intermediate transfer line between the firstintermediate transfer line and the extract stream transfer line awayfrom the adsorptive separation chamber to remove residual fluid fromintermediate transfer line. The process also includes directing theresidual fluid flushed from the intermediate transfer line to a recyclestream to introduce the residual fluid into the adsorptive separationchamber. In this manner an amount of fluid required by the process maybe reduced.

According to one approach, the process includes transferring residualfluid flushed from the intermediate transfer line to a bottoms portionof a raffinate fractionation column to be sent to the recycle stream.According to another approach, the process includes transferring theresidual fluid flushed from the intermediate transfer line a bottomsportion of an extract fractionation column to be sent to the recyclestream. According to these approaches, the residual fluid is not heatedto an extract column bottoms outlet temperature thereby reducing energyconsumption.

According to another approach, a process is provided for the separationof components in a feed stream comprising at least one preferentiallyadsorbed component and at least one non-preferentially adsorbedcomponent by simulated countercurrent adsorptive separation thatincludes introducing a feed stream into a port of a multi-bed adsorbentchamber comprising a plurality of ports with corresponding transferlines via a transfer line in fluid communication with the port. Theprocess also includes flushing residual feed from the transfer line intothe adsorptive separation chamber with a flushing fluid to fill thetransfer line with the flushing fluid. The process according to thisapproach further includes flushing residual flushing fluid in thetransfer line away from the adsorptive separation chamber with fluidfrom a purification zone of the adsorptive separation chamber adjacentto the port to fill the transfer line with the purification zone fluid.The process also includes withdrawing an extract stream from theadsorptive separation chamber through the transfer line that has ahigher concentration of the preferentially adsorbed component than thefeed stream and a lower concentration of the non-preferentially adsorbedcomponent than the feed stream. In this manner, the transfer line isfilled with purification zone fluid having a similar composition to theextract stream prior to withdrawal of the extract stream therethrough torestrict contamination of the extract stream with the non-preferentiallyadsorbed component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 2 is a compositional diagram of fluid within a simulated-moving-bedadsorptive separation chamber in accordance with various embodiments ofthe invention;

FIG. 3 is a perspective view of a rotary valve in accordance withvarious embodiments of the invention;

FIGS. 4-6 are graphs illustrating the volumetric flow rate of fluidthrough transfer lines in accordance with various embodiments of theinvention; and

FIG. 7 is a simplified diagram of a Prior Art simulated-moving-bedadsorption process.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments of the present invention. It will further beappreciated that certain actions and/or steps may be described in aparticular order of occurrence while those skilled in the art willunderstand that such specificity with respect to sequence is notactually required. It will also be understood that the terms andexpressions used herein have the ordinary technical meaning as isaccorded to such terms and expressions by persons skilled in thetechnical field as set forth above except where different specificmeanings have otherwise been set forth herein.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

Co-pending unpublished U.S. patent application Ser. No. 13/630,461describes a process wherein a first portion of the raffinate stream isdirected to a recycle line and a second portion is directed to theraffinate column. As the transfer line used for withdrawing theraffinate stream was previously occupied by the desorbent stream,residual desorbent remains in the transfer line. By directing this firstportion of the raffinate stream away from the raffinate column,separation of this residual desorbent in the raffinate column and theassociated energy penalty could be avoided. It has been identified,however, that in some systems, when no surge capacity in the feed to theraffinate column is available, the feed to the column may bediscontinuous and compromise steady column control.

Adsorptive separation is applied to the recovery of a variety ofhydrocarbon and other chemical products. Chemical separations using thisapproach which have been disclosed include the separation of mixtures ofaromatics into specific aromatic isomers, of linear from nonlinearaliphatic and olefinic hydrocarbons, of either paraffins or aromaticsfrom a feed mixture comprising both aromatics and paraffins, of chiralcompounds for use in pharmaceuticals and fine chemicals, of oxygenatessuch as alcohols and ethers, and of carbohydrates such as sugars.Aromatics separations include mixtures of dialkyl-substituted monocyclicaromatics and of dimethyl naphthalenes. A major commercial application,which forms the focus of the prior references and of the followingdescription of the present invention without so limiting it, is therecovery of para-xylene and/or meta-xylene from mixtures of C₈aromatics, due to typically high purity requirements for these products.Such C₈ aromatics usually are derived within an aromatics complex by thecatalytic reforming of naphtha followed by extraction and fractionation,or by transalkylation or isomerization of aromatics-rich streams in suchcomplexes; the C₈ aromatics generally comprise a mixture of xyleneisomers and ethylbenzene. Processing of C₈ aromatics usingsimulated-moving-bed adsorption generally is directed to the recovery ofhigh-purity para-xylene or high-purity meta-xylene; high purity usuallyis defined as at least 99.5 wt.-% of the desired product, and preferablyat least 99.7 wt.-%. It should be understood, that while the followingdetailed description focuses on the recovery of high-purity para-xylenefrom a mixed xylene and ethylbenzene stream, the invention is not solimited, and is also applicable for separating other components from astream comprising two or more components. As used herein, the termpreferentially adsorbed component refers to a component or components ofa feed stream that are more preferentially adsorbed than one or morenon-preferentially adsorbed components of the feed stream.

The invention normally is employed in an adsorptive separation processwhich simulates countercurrent movement of the adsorbent and surroundingliquid as described above, but it may also be practiced in a cocurrentcontinuous process, like that disclosed in U.S. Pat. Nos. 4,402,832 and4,478,721. The functions and properties of adsorbents and desorbents inthe chromatographic separation of liquid components are well-known, andreference may be made to U.S. Pat. No. 4,642,397, which is incorporatedherein, for additional description of these adsorption fundamentals.Countercurrent moving-bed or simulated-moving-bed countercurrent flowsystems have a much greater separation efficiency for such separationsthan fixed-bed systems, as adsorption and desorption operations arecontinuously taking place with a continuous feed stream and continuousproduction of extract and raffinate. A thorough explanation ofsimulated-moving-bed processes is given in the Adsorptive Separationsection of the Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY at page563.

FIG. 1 is a schematic diagram of a simulated-moving-bed adsorptionprocess in accordance with one aspect. The process sequentially contactsa feed stream 5 with adsorbent contained in the vessels and a desorbentstream 10 to separate an extract stream 15 and a raffinate stream 20. Inthe simulated-moving-bed countercurrent flow system, progressiveshifting of multiple liquid feed and product access points or ports 25down an adsorbent chamber 100 and 105 simulate the upward movement ofadsorbent contained in the chamber. The adsorbent in asimulated-moving-bed adsorption process is contained in multiple beds inone or more vessels or chambers; two chambers 100 and 105 in series areshown in FIG. 1, although a single chamber 902 as illustrated in FIG. 7or other numbers of chambers in series may be used. Each vessel 100 and105 contains multiple beds of adsorbent in processing spaces. Each ofthe vessels has a number of ports 25 relating to the number of beds ofadsorbent, and the position of the feed stream 5, desorbent stream 10,extract stream 15 and raffinate stream 20 are shifted along the ports 25to simulate a moving adsorbent bed. Circulating liquid comprisingdesorbent, extract and raffinate circulates through the chambers throughpumps 110 and 115, respectively. Systems to control the flow ofcirculating liquid are described in U.S. Pat. No. 5,595,665, but theparticulars of such systems are not essential to the present invention.A rotary disc type valve 300, as characterized for example in U.S. Pat.Nos. 3,040,777 and 3,422,848, effects the shifting of the streams alongthe adsorbent chamber to simulate countercurrent flow. Although therotary disc valve 300 is described herein, other systems and apparatusfor shifting the streams along the adsorbent chamber are alsocontemplated herein, including systems utilizing multiple valves tocontrol the flow of the streams to and from the adsorbent chamber 100and/or 105 as for example, described in U.S. Pat. No. 6,149,874.

Referring to FIG. 3, a simplified exploded diagram of an exemplaryrotary valve 300 for use in an adsorptive separation system and processis depicted. A base plate 474 includes a number of ports 476. The numberof ports 476 equal the total number of transfer lines to the chamber(s)100 and 105. The base plate 474 also includes a number of tracks 478.The number of tracks 478 equal the number of net input, output, andflush lines for the adsorptive separation unit (not shown in FIG. 3, butillustrated as 5′, 10′, 15′, 20′, and 35′ in FIG. 1). The net inputs,outputs, and flush lines are each in fluid communication with adedicated track 478. Crossover lines 470 place a given track 478 influid communication with a given port 476. In one example, the netinputs include a feed input and a desorbent input, the net outputsinclude an extract output and a raffinate output, and the flush linesinclude between about one and about four flush lines. As the rotor 480rotates as indicated each track 478 is placed in fluid communicationwith the next successive port 476 by crossover line 470. A seal sheet472 is also provided. It should be noted that FIG. 3 genericallyrepresents a rotary valve of the type that can be used in the presentsystem and process, however it should not be understood that the numberof tracklines 478, crossover lines 470, etc., corresponds to the numberthat would be present in any given system as the specific rotary valvedesign depends on the total number of net stream to and from the rotaryvavle, which may be different, for example, depending upon the lineflushing protocol that is utilized and the number of flushing streamsthat are present.

The various streams involved in simulated-moving-bed adsorption asillustrated in the figures and discussed further below with regard tothe various aspects of the invention described herein may becharacterized as follows. A “feed stream” is a mixture containing one ormore extract components or preferentially adsorbed components and one ormore raffinate components or non-preferentially adsorbed components tobe separated by the process. The “extract stream” comprises the extractcomponent, usually the desired product, which is more selectively orpreferentially adsorbed by the adsorbent. The “raffinate stream”comprises one or more raffinate components which are less selectivelyadsorbed or non-preferentially adsorbed. “Desorbent” refers to amaterial capable of desorbing an extract component, which generally isinert to the components of the feed stream and easily separable fromboth the extract and the raffinate, for example, via distillation.

The extract stream 15 and raffinate stream 20 from the illustratedschemes contain desorbent in concentrations relative to the respectiveproduct from the process of between 0% and 100%. The desorbent generallyis separated from raffinate and extract components by conventionalfractionation in, respectively, raffinate column 150 and extract column175 as illustrated in FIG. 1 and recycled to a stream 10′ by raffinatecolumn bottoms pump 160 and extract column bottoms pump 185 to bereturned to the process. FIG. 1 shows the desorbent as bottoms from therespective column, implying that the desorbent is heavier than theextract or raffinate; different commercial units for the separation ofC₈ aromatics employ either light or heavy desorbents, and thus in someapplications the desorbent may be separated at a different locationalong the fractionation columns 150 and 175. The raffinate product 170and extract product 195 from the process are recovered from theraffinate stream and the extract stream in the respective columns 150and 175; the extract product 195 from the separation of C₈ aromaticsusually comprises principally one or both of para-xylene andmeta-xylene, with the raffinate product 170 being principallynon-adsorbed C₈ aromatics and ethylbenzene.

The liquid streams, e.g., the streams of feed 5, desorbent 10, raffinate20, and extract 15 entering and leaving the adsorbent chambers 100 and105 via the active liquid access points or ports 25 effectively dividethe adsorbent chamber 100 and 105 into separate zones which move as thestreams are shifted along the ports 25. It should be noted that whilemuch of the discussion herein refers to FIG. 1 and the location of thestreams in FIG. 1, FIG. 1 illustrates only a current location of thestreams at a single step or a snapshot of the process as the streamstypically shift downstream at different steps of a cycle. As the streamsshift downstream, the fluid composition and the corresponding zonesshift downstream therewith. In one approach, the position of the streamswith regard to the access points or ports 25 of the adsorptiveseparation chambers 100 and 105 remain generally constant with regard toone another as they synchronously progress downstream along the ports25. In one example, the streams each progress a single port 25downstream for each step and each stream occupies each port 25 one timeduring an entire cycle. According to one example, the streams arestepped simultaneously to subsequent ports 25 by rotating a rotary valve300, and are maintained at a particular port 25 or step for apredetermined step-time interval. In one approach, there are betweenabout 4 and 100 ports 25, between about 12 and 48 ports in anotherapproach, and between about 20 and 30 ports in yet another approach, andan equal number of corresponding transfer lines. In one example, theadsorptive separation chamber or chambers 100 and 105 include about 24ports and each stream is shifted to each of the 24 ports 25 during acomplete cycle so that each stream occupies each port 25 andcorresponding transfer line during the cycle. In this example, a cyclemay be between about 20 and about 40 minutes in one approach and betweenabout 22 and 35 minutes in another approach. In one approach, astep-time interval is between about 30 seconds and about two minutes. Inanother approach, the step-time interval is between about 45 seconds andabout one minute thirty seconds. In yet another approach, the step-timeinterval is between about 50 seconds and about one minute and 15seconds. An example of a typical step-time interval may be about 1minute.

With this in mind, FIG. 2 illustrates a snapshot of the compositionalprofile of the fluid within an adsorptive separation chamber (a singleadsorptive separation chamber 100 is illustrated in FIG. 2 forsimplicity) at one point during a step and the corresponding zones intowhich the adsorptive separation chamber 100 is divided. The adsorptionzone 50 is located between the feed inlet stream 5 and the raffinateoutlet stream 20. In this zone, the feed stream 5 contacts theadsorbent, an extract component is adsorbed, and a raffinate stream 20is withdrawn. As illustrated in the figure, the raffinate stream 20 maybe withdrawn at a location where the composition includes raffinatefluid 454 and little if any extract fluid 450. Immediately upstream withrespect to fluid flow is the purification zone 55, defined as theadsorbent between the extract outlet stream 15 and the feed inlet stream5. In the purification zone 55, the raffinate component is displacedfrom the nonselective void volume of the adsorbent and desorbed from thepore volume or surface of adsorbent shifting into this zone by passing aportion of extract stream material leaving the desorption zone 60. Thedesorption zone 60, upstream of the purification zone 55, is defined asthe adsorbent between the desorbent stream 10 and the extract stream 15.The desorbent passing into this zone displaces the extract componentwhich was adsorbed by previous contact with feed in the adsorption zone50. The extract stream 15 may be withdrawn at a location of the chamber100 that includes extract fluid 450 and little if any raffinate fluid454. A buffer zone 65 between the raffinate outlet stream 20 and thedesorbent inlet stream 10 prevents contamination of the extract, in thata portion of the desorbent stream enters the buffer zone to displaceraffinate material present in that zone back into the adsorption zone50. The buffer zone 65 contains enough adsorbent to prevent raffinatecomponents from passing into the desorption zone 60 and contaminatingthe extract stream 15.

Each of the zones described above generally are effected throughmultiple compartments or “beds” as described in U.S. Pat. No. 2,985,589.The positions of the various streams described are structurallyseparated from one another by a horizontal liquidcollection/distribution grid. Each grid is connected to a transfer linedefining a transfer point at which process streams enter and leave theadsorbent chamber. This arrangement facilitates the distribution offluids within the chamber through eliminating channeling and otherinefficiencies, prevents convective back-mixing of fluid in a directionopposite to that of primary fluid flow, and prevents migration ofadsorbent through the chamber. Each of the zones described above usuallycomprises a plurality of 2 to 10, and more usually 3 to 8, beds. Atypical simulated-moving-bed adsorption unit comprises 24 beds ofadsorbent.

It is readily apparent in FIG. 1 that when a transfer line at an accesspoint 25 which is being used to transport a particular stream into orout of the adsorbent chamber is left idle at the end of a step it willremain full of the compounds forming that stream until these compoundsare removed from the line by a second flowing stream. In this regard, itshould be noted that only active transfer lines, i.e. those linespresently facilitating flow of fluid therethrough, are illustrated inFIG. 1, although intermediate transfer lines are present at each of theports 25 along the chambers 100 and 105 to facilitate fluid flow uponshifting of the fluid streams to subsequent ports 25. The residual fluidor compounds left in the now unused transfer line after a stream shiftsto a subsequent transfer line, will therefore be either withdrawn fromthe process as the initial part of a process stream removed from theprocess or forced into the adsorbent chamber when the transfer linecarries a stream into the adsorbent chamber. To further understanding,FIG. 7 is included and illustrates a previous system showing unusedtransfer lines as dashed lines and transfer lines currently occupied bya stream, e.g. stream 920 as solid lines extending from ports of theadsorptive separation chamber 902.

Returning to FIG. 1, as described above, the presence of residual fluidin the transfer lines can have negative effects on the performance of asimulated-moving-bed adsorptive separation process. For example,residual raffinate in a transfer line which previously had been used toremove the raffinate stream 20 from the adsorbent chamber may be flushedinto the adsorbent chamber 105 with the feed stream 5 when it shifts tothat transfer line in a subsequent step. Similarly, residual feed in atransfer line which previously had been used to introduce the feedstream 5 to the adsorbent chamber may be removed from the transfer linewith the extract stream 15 when it shifts to that transfer line in asubsequent step. Likewise, residual extract in a transfer line whichpreviously had been used to remove the extract stream from the adsorbentchamber may be flushed back into the adsorbent chamber 100 with thedesorbent stream 10 when it subsequently arrives at that transfer line.

It has been identified that residual fluid, including desorbentremaining in an intermediate transfer line between the transfer linecurrently occupied by the desorbent stream 10 and the transfer linecurrently occupied by the raffinate stream 20 after the desorbent stream10 is stepped to a subsequent transfer line can cause an energy penaltyduring separation of the raffinate. More particularly, as noted above,when the raffinate stream 20 shifts to the intermediate transfer lineand raffinate is withdrawn therethrough, the residual desorbent in thetransfer line will be transferred with the raffinate stream 20 alongline 20′ to the inlet of the raffinate column 150. The desorbent will beheated to the column inlet temperature before being separated from theraffinate product 170 during fractionation. Thus, the residual desorbentis heated to the inlet temperature and cooled back to the originaltemperature requiring an energy expenditure, without providing a benefitin terms of additional raffinate that is separated, since this residualfluid should have none or very little of the raffinate components in it.Further, it has been identified that the surges of fluid to the columnthat include very little of the raffinate components, e.g. xylenes, candisturb steady state operation of the column 170.

Turning to FIG. 1, an adsorptive separation system and process inaccordance with one aspect is illustrated. According to this aspect, adesorbent flush stream 35 is provided for removing residual fluid from atransfer line that will subsequently be used to withdraw the raffinatestream 20. The desorbent flush stream 35 flushes residual fluid from thetransfer line 45 away from the adsorptive separation chamber 105 priorto the withdrawal of the raffinate stream 20 through the transfer line.It should be noted that while the desorbent flush stream 35 isillustrated as removing fluid from the transfer line 45 adjacent to aportion of adsorptive separation chamber 105, during another step of theprocess the desorbent flush stream 35 will be used to remove residualfluid from a transfer line to adsorptive separation chamber 100. Itshould also be noted that less than two or more than two adsorptiveseparation chambers may be utilized within the scope of the descriptionherein.

The desorbent flush stream 35 flushes the residual fluid from thetransfer line 45 toward a destination other than the inlet to theraffinate column 150. Advantageously, in this manner, the residual fluidin the transfer line 45 is not mixed with the raffinate stream 20 duringwithdrawal of the raffinate stream 20 through the transfer line 45during a subsequent step and sent to the inlet of the raffinate column150. The first destination may be a recycle line 10′ for recycling theraffinate stream and the portion of the residual fluid to the adsorptiveseparation chamber 100. In this regard, by recycling a portion of thefluid back to the adsorptive separation chamber 100 the amount of fluidprocessed by the raffinate fractionation column 150 is reduced.

Because the residual fluid in the transfer line will contain a greaterpercentage of desorbent than the raffinate stream fluid, this excessdesorbent is advantageously not sent to and separated in the raffinatefractionation column 150. Because fluid entering the raffinatefractionation column inlet 165 is heated in the column, if the excessdesorbent in the residual fluid was introduced into the raffinatefractionation column 150 it would be heated without providing additionalyield of the extract product, and thus incurring an energy penalty.Thus, by diverting the initial slug of fluid so that excess desorbent isnot introduced into the raffinate fractionation column 150, the amountof energy required by the system is reduced.

Turning to more of the particulars, according to one aspect thedesorbent flush stream 35 flushes the residual fluid in the transferline 45 away from the adsorptive separation chamber 100. It should benoted that the transfer line 45 is used for the desorbent flush stream35 during the step interval illustrated in FIG. 1, however, duringprevious or subsequent steps the desorbent flush stream 35 will shiftalong with the other streams to subsequent transfer lines and be used toremove residual fluid from the subsequent transfer lines.

In one approach, desorbent flush stream 35 is positioned at anintermediate transfer line 45 between desorbent stream 10 and theraffinate stream 20. In this regard, in order to flush the residualfluid from the transfer line 45, fluid from adsorbent chamber 100, andmore specifically, the buffer zone 65, adjacent to the transfer lineport 45′ corresponding to the transfer line 45 is withdrawn from theadsorbent chamber 100 and used to flush the residual fluid away from theadsorbent chamber 100 and 105. The desorbent flush stream 35 may then betransferred for further processing, including recycled to the adsorptiveseparation chamber via line 10′.

Because this fluid from the buffer zone 65 is similar in composition tothe raffinate stream 20 that will be subsequently withdrawn from thetransfer line 45, the residual fluid remaining in the bed line after thedesorbent flush will advantageously be similar in composition to thedesired raffinate stream composition. As there may be several transferlines in fluid communication with the purification zone between thedesorbent stream 10 transfer line and the raffinate stream 20 transferline, it may be advantageous to flush the residual fluid including theresidual desorbent away from an intermediate transfer line using bufferzone fluid that is near the raffinate stream 20 transfer line. In thismanner, the buffer zone 65 fluid used to flush the residual fluid fromthe transfer line 45 may be more similar in composition to the raffinatestream 20 than if the desorbent flush was conducted on a transfer linethat was closer to the transfer line currently occupied by the desorbentstream 10. To this end, in one example, the desorbent flush stream 35 iswithin two transfer lines from the transfer line currently occupied bythe raffinate stream 20, and more preferably, by another example, withinone transfer line from the transfer line currently occupied by theraffinate stream 20.

In one example, the desorbent stream 10 includes more than about 90%desorbent and in another example more than about 95% desorbent. In oneexample the desorbent stream 10 includes less than about 10% of thenon-preferentially adsorbed components and less than about 1% of thenon-preferentially adsorbed components in another example. In oneexample, the fluid withdrawn from the buffer zone 65 used to flush theresidual fluid from the transfer line 45 may include between about 90%and about 95% desorbent and between about 1% and about 5%non-preferentially adsorbed components; and in another example, betweenabout 95% and about 99% desorbent and between about 1% and about 5%non-preferentially adsorbed components.

In one approach, the desorbent flush stream 35 is passed through line35′ to a fluid recycle line 10′. The fluid recycle line 10′ may includeprimarily desorbent that is separated via the fractionation columns 150and 175 and recycled back to the adsorptive separation chamber 100 whereit is reused in the process. In one approach, the desorbent flush streamis sent via line 35′ to a bottoms portion 155 of the raffinatefractionation column 150 where it is combined with the desorbentseparated by the raffinate fractionation column 150 and sent to thefluid recycle line 10′ via a raffinate bottoms pump 160. In anotherapproach the secondary flush stream is sent via line 35′ to a bottomsportion 180 of the extract fractionation column 175 where it is combinedwith the desorbent separated by the extract fractionation column 175 andsent to the fluid recycle line 10′ via a extract bottoms pump 185. Inyet another approach, first and second portions of the desorbent flushstream may be sent via line 35′ to the bottoms portion 155 of theraffinate fractionation column 150 and the bottoms portion 180 of theextract fractionation column 175 respectively.

Further, in one approach, after the desorbent flush of transfer line 45,during a subsequent intermittent step when the raffinate stream 20 iswithdrawn through the transfer line 45, fluid within the transfer line45 will be withdrawn with the raffinate stream 20 and sent to theraffinate fractionation column 150 to be separated via a distillation.The residual fluid within the transfer line 45 after the desorbent flushthat is sent with the raffinate stream 20 to the raffinate fractionationcolumn 150 is heated with the raffinate stream 20. Because this residualfluid is more similar in composition to the raffinate stream 20 than theresidual fluid previous flushed from transfer line 45, fractionation ofthis fluid will result in increased recovery of the desired raffinateproduct 170. Thus, unlike prior systems, fluid remaining in the transferline 45 after the desorbent flush that is subsequently taken up with theraffinate stream 20 and sent to the raffinate fractionation column 150will not result in an unnecessary utilities penalty, becausedistillation of this fluid will result in additional yield of theraffinate product 195 rather than primarily desorbent to be separatedout of the column bottoms 155.

In one approach, the desorbent flush stream 35 is withdrawn from theadsorptive separation chamber 100 or 105 and sent along a transfer line45. In one approach a rotary valve 300 is provided so that the desorbentflush stream 35 is withdrawn through the transfer line 45 and directedto the rotary valve 300. According to this approach, a dedicateddesorbent flush stream trackline 478 may be provided, wherein thedesorbent flush stream fluid is passed to the desorbent flush net streamline 35′. The residual desorbent net stream flush line 35′ may be influid communication with the desorbent net line 10′ in order to recyclethe residual desorbent to the adsorptive separation chambers 100 and105. To this end, the residual desorbent net stream flush line 35′ maybe in communication with one or both of the extract column bottomsportion 180 and the raffinate column bottoms portion 155, wherein theresidual desorbent can be combined with the bottoms and recycled vialine 10′ back to the adsorptive separation chambers 100 and 105. Otherconfigurations without a rotary valve 300 are also contemplated herein,such as providing a dedicated residual desorbent flush net stream linefor each transfer line of the adsorptive separation chambers 100 and105.

As described previously, in accordance with various aspects of theinvention, countercurrent adsorptive separation includes introducing afeed stream 5, comprising at least one preferentially adsorbed componentand at least one non-preferentially adsorbed component, and a desorbentstream 10 into two different ports 25 via two different correspondingtransfer lines along a multi-bed adsorptive separation chamber having aplurality of beds that are serially connected in fluid communication andcomprising a predetermined number of spaced ports with correspondingtransfer lines in fluid communication therewith for introducing andremoving fluid into and from the adsorptive separation chamber andwithdrawing an extract stream 15 and raffinate stream 20 through twodifferent ports of the multi-bed adsorptive separation chamber via twodifferent corresponding transfer lines. The various streams that areintroduced and withdrawn to and from the adsorptive separation chamber100 and 105 are sequentially shifted or stepped downstream to subsequentports. The various streams are typically stepped simultaneously tosubsequent ports 25, for example by rotating the rotary valve 300, andare maintained at a particular port 25 or step for a predeterminedstep-time interval. As discussed above, in one approach, there arebetween about 4 and 100 ports 25, between about 12 and 48 ports inanother approach, and between about 20 and 30 ports in yet anotherapproach, and an equal number of corresponding transfer lines. In oneexample, the adsorptive separation chamber or chambers 100 and 105include about 24 ports and each stream is shifted to each of the 24ports 25 during a complete cycle so that each stream occupies each port25 and corresponding transfer line during the cycle. In this example, acycle may be between about 20 and about 40 minutes in one approach andbetween about 22 and 35 minutes in another approach. In one approach, astep-time interval is between about 30 seconds and about two minutes. Inanother approach, the step-time interval is between about 45 seconds andabout one minute thirty seconds. In yet another approach, the step-timeinterval is between about 50 seconds and about one minute and 15seconds.

In this regard, the process may include flushing the intermediatetransfer line between two lines currently occupied by two of the typicalstreams, including the feed stream 5, the desorbent stream 10, theextract stream 15, and the raffinate stream 20 at a non-uniform ordynamic volumetric flow rate during the step-time interval. According toone aspect the process includes flushing the intermediate transfer line45 at a first flow rate during a first portion of the step-timeinterval. The process includes flushing the intermediate transfer lineat a second flow rate during a second portion of the step-time intervallater during the step-time interval than the first portion. In thismanner, a greater volume of fluid is flushed from the intermediatetransfer line during one of the first and second portion of thestep-time interval than during the other portion. Flushing the transferline at a non-constant flow rate may provide performance advantages interms of the composition of fluid flushed into or from the intermediatetransfer line as well as the timing of introducing fluids to or from theintermediate transfer line.

In one aspect, the non-constant flow rate may include a ramped orexponentially increasing or decreasing flow rate that increases ordecreases during at least a portion of the step-time interval. In thisregard, the ramped flow rate may increase or decrease during a portionof the step-time interval and may vary linearly or non-linearly, e.g.exponentially during that time. By another aspect, the non-constant flowrate may include step increases or decreases in the flow rate so thatone or both of the first flow rate and the second flow rate is constantand one is different than the other of the first flow rate and thesecond flow rate. In yet another aspect, the non-constant flow rate mayinclude a combination of ramped portions and step increases anddecreases in the volumetric flow rate. The non-constant flow rate mayalso include additional flow rates during additional portions of thestep-time interval. The flow rate may increase, decrease, or remainunchanged during any particular step. In addition the flow rate may bechanged from the initial value to a higher value, lower value or zero atthe conclusion of a step. Further one of the flowrates during a steptime interval may be a zero flowrate so that little or no fluid flowsthrough the intermediate transfer line during that portion of the steptime interval. FIGS. 4-6 illustrate examples of non-constant flow ratesin accordance with various aspects. FIG. 4 illustrates a ramped flowrate 1015 that increases over time 1020 during at least a portion of thestep-time interval. In this example, a first flow rate 1005 is lowerthan a second flow rate 1010 so that a greater volume of fluid isflushed during the second portion of the step-time interval than duringthe first portion. In another example, the ramped flow rate decreasesover time so that a first flow rate is higher than a second flow rate sothat a greater volume of fluid is flushed during the first portion ofthe step-time interval than during the second portion. FIG. 5 on theother hand illustrates an example of a non-constant stepped flow rate.In this example, the flow rate 1115 is at a first generally constantflow rate 1105 during a first portion of the step-time interval 1120 andincreases to a second and generally constant higher flow rate 1110during the second portion of the step-time interval 1120. In anotherexample, the stepped flow rate has a second generally constant flow rateduring the second portion of the step-time interval that is lower than afirst flow rate so that so that more volume of fluid is flushed duringthe first potion of the step-time interval. The volumetric flow rateduring one of the first and second portions may be zero according tovarious aspects. In yet another example, illustrated in FIG. 6, the flowrate 1215 at a first portion of the step-time interval 1220 begins at afirst flow rate 1205 and then includes second flow rate 1210 thatexponentially decreases over time during a second portion of thestep-time interval 1220. Other flow rate profiles are also contemplatedin accordance with various aspects of the invention that have differentfirst and second flow rates during corresponding first and secondportions of the step-time interval and may there may be additionalportions of the step-time interval with still other flow rates.

In accordance with one aspect, one of the first and second flow rates issufficient to flush between about 50% and 400% of the volume of theintermediate transfer line 45 and associated valving so that most or allof the residual fluid within the transfer line is flushed during thefirst or second portion of the step-time interval. According to anotheraspect, one of the first and second flow rates is sufficient to flushbetween about 75% and about 200% of the intermediate transfer line 45and associated valving volume during the first or second portion of thestep-time interval. In yet another aspect, one of the first and secondflow rates is sufficient to flush between about 90% and about 150% ofthe intermediate transfer line 45 and associated valving volume duringthe first or second portion of the step-time interval. The other of thefirst and second flow rates according to various aspects may besufficient to flush between about 0% and about 75% of the transfer lineand valving volume in one approach, between about 0% and about 50% ofthe transfer line and valving volume in another approach, and betweenabout 0% and about 25% of the transfer line valving volume in yetanother approach.

According to one aspect, the second flow rate is higher than the firstflow rate so that a greater volume of fluid is flushed during the secondportion of the step-time interval than during the first portion of thestep-time interval. The process according to this aspect may beparticularly useful where residual fluid is being flushed away from theadsorptive separation chamber 100 and 105 with flushing fluid withdrawnfrom the adsorptive separation chamber 100 and 105. In this regard, theflushing fluid is provided a greater dwell time within the adsorptiveseparation chamber than when a constant flow rate is used or when thefirst flow rate is greater than the second flow rate. Thisadvantageously provides for greater separation of components in theflushing fluid so that the flushing fluid will be more similar incomposition than a subsequent stream withdrawn from or introduced intothe adsorptive separation chamber 100 and 105.

In one example, the process includes flushing the intermediate transferline 45, which may include residual desorbent fluid from beingpreviously occupied by the desorbent stream 10, at a first volumetricflow rate during a first portion of the step-time interval that is lessthan a second volumetric flow rate during a second subsequent portion ofthe step-time interval. In this manner, as the flushing fluid withdrawnfrom the buffer zone 65 provided a longer amount of time in the bufferzone 65 so that it is more similar in composition to the raffinatestream 20 that will subsequently be withdrawn from the intermediatetransfer line 45 during a subsequent step. By dynamically altering theflowrate of the residual desorbent flush stream 35 in this manner, theresidual fluid remaining in the transfer line 45 after the flush willinclude a smaller amount of desorbent that must be passed to theraffinate column 150, further decreasing the energy penalty associatedwith separating the raffinate product 170 from the desorbent.

In accordance with various aspects, the volumetric flow rate of thefluid through the transfer lines during dynamic flushing thereof may becontrolled using valving and a controller. The valving may beincorporated into transfer lines themselves to control or restrict thevolumetric flow rate of fluid flowing therethrough. A controller may beprovided for controlling the valves and the flow rate of the fluidthrough the transfer lines. The valving may also be incorporated inother locations within the system, for example on the downstream side ofa rotary valve 300 when a rotary valve is incorporated or in downstreamlines for transferring the fluid to downstream components of the system,for example the lines 15′ and 20′ for transferring fluid to the extractfractionation column 175 or the raffinate fractionation column 150,respectively.

The system described herein for flushing residual fluid may be utilizedwith one or more additional flush protocols, for example a primary,secondary, or tertiary flush as described previously or one or more ofthe flush protocols described in Co-pending unpublished U.S. patentapplication Ser. No. 13/630,461. The flushing protocols may be selectedto improve the energy efficiency of the system and process as well as toimprove the separation process to meet separation or purityrequirements.

A variety of adsorbents may be employed for the presentsimulated-moving-bed process. The practice of the subject invention thusis not related to or limited to the use of any particular adsorbent oradsorbent/desorbent combination, as differing sieve/desorbentcombinations are used for different separations. The adsorbent may ormay not include a zeolite. Examples of adsorbents which may be used inthe process described herein include nonzeolitic molecular sievesincluding carbon-based molecular sieves, silicalite and the crystallinealuminosilicates molecular sieves classified as X and Y zeolites.Details on the composition and synthesis of many of these microporousmolecular sieves are provided in U.S. Pat. No. 4,793,984, which isincorporated herein for this teaching. Information on adsorbents mayalso be obtained from U.S. Pat. Nos. 4,385,994; 4,605,492; 4,310,440;and 4,440,871.

In adsorptive separation processes, which generally are operatedcontinuously at relatively constant pressures and temperatures forliquid phase operation, the desorbent material may be selected tosatisfy several criteria. First, the desorbent material should displacean extract component from the adsorbent with reasonable mass flow rateswithout itself being so strongly adsorbed as to unduly prevent anextract component from displacing the desorbent material in a followingadsorption cycle. Expressed in terms of the selectivity, it is generallypreferred that the adsorbent be more selective for the extractcomponents with respect to a raffinate components than it is for thedesorbent material with respect to a raffinate components. Secondly,desorbent materials should be compatible with the particular adsorbentand the particular feed mixture. More specifically, they should notreduce or destroy the capacity of the adsorbent or selectivity of theadsorbent for an extract component with respect to a raffinatecomponent. Additionally, desorbent materials should not chemically reactwith or cause a chemical reaction of either an extract component or araffinate component. Both the extract stream and the raffinate streamare typically removed from the adsorbent void volume in admixture withdesorbent material and any chemical reaction involving a desorbentmaterial and an extract component or a raffinate component or both wouldcomplicate or prevent product recovery. The desorbent should also beeasily separated from the extract and raffinate components, as byfractionation. The desorbent may include a heavy or light desorbentdepending on the particular application. In certain embodiments, a heavydesorbent is selected from the group consisting of para-diethylbenzene,para-diisopropylbenzene, tetralin, and the like, and combinationsthereof. In certain embodiments, toluene and the like can be used as thelight desorbent. The para-diethylbenzene (p-DEB) has a higher boilingpoint than the C8 aromatic isomers and, as such, the p-DEB is thebottoms (i.e., heavy) product when separated from the C8 isomers in afractional distillation column. Similarly, toluene has a lower boilingpoint than the C8 aromatic isomers and, as such, the toluene is theoverhead (i.e., light) product when separated from the C8 isomers in afractional distillation column. The p-DEB has become a commercialstandard for use as a desorbent in separations of para-xylene. Thus,while the above description describes the desorbent as being removed asa bottoms portion of the raffinate and extract fractionation columns, itshould be understood that the desorbent may be recovered as overhead ora side cut of the fractionation columns depending on the particulardesorbent used and the type of separation.

Adsorption conditions in general include a temperature range of fromabout 20° to about 250° C., with from about 60° to about 200° C. beingpreferred for para-xylene separation. Adsorption conditions also includea pressure sufficient to maintain liquid phase, which may be from aboutatmospheric to 2 MPa. Desorption conditions generally include the samerange of temperatures and pressure as used for adsorption conditions.Different conditions may be employed for other extract compounds.

The above description and examples are intended to be illustrative ofthe invention without limiting its scope. While there have beenillustrated and described particular embodiments of the presentinvention, it will be appreciated that numerous changes andmodifications will occur to those skilled in the art, and it is intendedin the appended claims to cover all those changes and modificationswhich fall within the true spirit and scope of the present invention.

The invention claimed is:
 1. A process for separating components in afeed stream by simulated countercurrent adsorptive separationcomprising: introducing a feed stream, comprising at least onepreferentially adsorbed component and at least one non-preferentiallyadsorbed component, and a desorbent stream into two different ports viatwo different corresponding transfer lines along a multi-bed adsorptiveseparation chamber having a plurality of beds that are seriallyconnected in fluid communication and comprising a predetermined numberof spaced ports with corresponding transfer lines in fluid communicationtherewith for introducing and removing fluid into and from theadsorptive separation chamber and withdrawing an extract stream andraffinate stream through two different ports of the multi-bed adsorptiveseparation chamber via two different corresponding transfer lines;flushing residual fluid comprising residual desorbent from anintermediate transfer line between the desorbent stream transfer lineand the raffinate stream transfer line away from the adsorptiveseparation chamber to remove the residual fluid from the intermediatetransfer line; directing the residual fluid flushed from theintermediate transfer line to a destination other than the inlet of araffinate column; and shifting the raffinate stream to the intermediatetransfer line and withdrawing the raffinate stream through theintermediate transfer line.
 2. The process of claim 1, wherein flushingthe residual fluid includes flushing the residual fluid with buffer zonefluid from the adsorptive separation chamber adjacent to theintermediate transfer line.
 3. The process of claim 2, wherein theintermediate transfer line is within two transfer lines of the transferline currently occupied by the raffinate stream.
 4. The process of claim2, wherein the buffer zone fluid has a lower concentration of desorbentthan the residual fluid.
 5. The process of claim 2, wherein theraffinate stream is withdrawn along with residual buffer zone materialwithin intermediate the transfer line, and passing the raffinate streamand the residual buffer zone material to a raffinate column.
 6. Theprocess of claim 1, wherein the one transfer line was previouslyoccupied by the desorbent stream so that the residual fluid within theone transfer line comprises primarily desorbent.
 7. The process of claim6, wherein the residual fluid destination is a desorbent recycle linefor recycling desorbent to the desorbent stream to be introduced intothe adsorptive separation chamber.
 8. The process of claim 1, whereinthe feed stream, the desorbent stream, the extract stream, the raffinatestream, and the residual fluid flush stream are sequentially shifted tosubsequent ports and their corresponding transfer lines along thepredetermined number of spaced ports and corresponding transfer linesand the intermediate transfer line was previously occupied by thedesorbent stream before the raffinate stream is withdrawn therethroughso that the residual fluid includes primarily desorbent.
 9. The processof claim 8, wherein a rotary valve provides fluid communication betweenthe intermediate transfer line and the recycle line during flushing ofthe intermediate transfer line, and shifting the rotary valve to asubsequent position to remove residual fluid from a subsequent transferline.
 10. The process of claim 9 wherein the rotary valve includes adedicated residual desorbent flush trackline, and passing the residualdesorbent stream from the intermediate transfer line, to the residualdesorbent flush trackline, and to the destination.
 11. The process ofclaim 1, wherein the flushing the residual fluid from the intermediatetransfer line is done during a step time interval and further includesflushing the fluid at a first flowrate during a first portion of thestep time interval and flushing the fluid at a second greater flowrateduring a second portion of the step time interval.